1. Field of the Invention
This invention relates to improvements in methods and circuits for determining the velocity of a data detector mechanism of a mass storage device, or the like, and more particularly to improvements in methods and circuits for generating a BEMF voltage which may be used in a feedback loop of a voice coil for determining the velocity of a data detector mechanism of a mass storage device, or the like.
2. Relevant Background
In mass storage devices, such as computer hard disk drives, CD-ROMs, DVDs, or the like, typically a recording and/or reading head or data detector is used that is positioned at selective locations radially outwardly from the center of a rotating data medium to read and/or write digital data onto the data medium. The head is positioned by selectively applying controlled voltages to a "voice coil" to move a mechanism that carries the head.
The head typically "flies" across the data medium, with air currents generated on account of the disk rotation causing the head to float above the surface of the medium. Under certain circumstances, the head is positioned to a "parked" position, typically with the head mechanism moved up a ramp or keeper assembly, so that when the disk rotation is stopped, the head is maintained at a position away from the recording medium. This is particularly important in "laptop" or portable computers that may be subjected to bumps or vibrations that might otherwise cause unwanted contact between the heads and the surface of the recording medium, which may damage either.
In moving the head to the ramped or parked position, it is important that sufficient voltage be applied to the voice coil to cause the head mechanism to be sufficiently moved to the parked position. At the same time, it is often important that an excessive voltage not be applied, since the parking is often performed during power down operations when power needs to be conserved for other power down functions and to insure that the heads are not overdriven into their parked position to avoid unnecessary damage. Consequently, it is desirable that a positioning voltage be applied to the voice coil in proportion to the instantaneous velocity of the head mechanism, so that just the correct energy is applied as may be necessary to move the head to the desired parked (or other) position.
Determining the instantaneous velocity of the head mechanism is difficult, however, since information is not instantaneously available from the driving voltages from which instantaneous velocity can be determined. It has been suggested that a back electromagnetic field (BEMF) voltage, which is proportional to the velocity, be used to develop the velocity information for use in a feedback system. This feedback is important to close the loop and ensure an accurate velocity control.
Typically the BEMF information is extracted by a circuit of the type shown in FIG. 1, described below, which performs the following operation: EQU V.sub.BEMF =V.sub.coil -I.sub.coil .times.Z.sub.coil
Where:
V.sub.coil is the voltage across the motor coil PA1 I.sub.coil is the current through the motor coil PA1 Z.sub.coil is the impedance of the motor coil (typically the square root of the sum of the squares of resistance plus inductive reactance)
While usually the voltage and current through the coil can be measured quite accurately, the difficulty of extracting an accurate value for the BEMF is due to the difficulty in estimating the value of the impedance of the coil. The impedance is especially hard to estimate because of the changes it undergoes due to production spread and due to temperature variations.
In a continuous closed loop system, often the desired impedance estimate accuracy is obtained through periodic "recalibration" of the value, which usually requires both hardware and software to perform the action. This often is expensive as a system solution, because it required digital to analog convertors and/or auto-zero circuits.
Thus, with reference now to FIG. 1, a circuit 10 is shown that may be used as a part of a control circuit for controlling the movement of the head mechanism (not shown) associated with the voice coil 11. A sense resistor 14 is connected in series with the coil 11 to receive driving currents that are applied via an input amplifier 16. The voltages across the coil 11 and resistor 14 are connected to a velocity detector 18, which generates at its output a BEMF voltage on line 20, which is summed with the input voltage to the circuit on line 22 by a summer circuit 24. The output of the summer circuit 24 is applied to the input of the amplifier 16. The circuit 10, therefore, provides driving currents to the coil 11 to move the head mechanism at a velocity determined by the velocity detector 18.
FIG. 2 shows one circuit which has been used to provide a velocity detector in the circuit of FIG. 1 in controlling the velocity of the motor using a BEMF voltage. The circuit 10 has a coil 11, which is illustrated having two parts, a pure inductive reactance part 12 and a resistance part 13. A current, I.sub.coil, is applied to the coil 11, to pass therethrough and through a sense resistor 14. The BEMF voltage on line 20 is developed by a first amplifier 22 that is connected across the coil 11, and a second amplifier 26, which is connected across the sense resistor 14. The outputs of the amplifiers 22 and 26 are summed in the summer circuit 24, the sum representing the BEMF voltage across the coil 11 which is produced on output line 20. It should be noted that the amplifiers 22 and 26 and summer circuit 24 form the velocity detector 18.